1. Field of the Invention
The present invention relates to a sensorless motor control circuit and, more particularly, to a sensorless motor control circuit capable of accurately determining a zero-crossing event of the back electrical motion force (BEMF) of a floated coil without employing any mask process.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing a conventional three-phase motor 11 and a switching circuit 12. The motor 11 has three coils A, B, and C, each of which has a terminal coupled together as a central point N, and another terminal Pa, Pb, or Pc respectively coupled to the switching circuit 12. The switching circuit 12 has three identical pairs of high-side and low-side switches SH1 to SH3 and SL1 to SL3, for making the terminals Pa, Pb, and Pc coupled to a supply voltage source Vm, or coupled to a ground potential, or even floated without being coupled to either the supply voltage source Vm or the ground potential.
More specifically, the switching circuit 12 is operated among six phases in sequence so as to complete a rotation cycle, for appropriately channeling the drive current through the three coils A, B, and C so as to effectively drive the motor 11. During the first phase, only are the first high-side switch SH1 and the second low-side switch SL2 turned on such that the terminal Pa of the coil A is coupled to the supply voltage source Vm, the terminal Pb of the coil B is coupled to the ground potential, and the terminal Pc of the coil C is floated. As a result, the drive current flows from the supply voltage source Vm, through the coils A to B in sequence, finally to the ground potential. During the second phase, only are the first high-side switch SH1 and the third low-side switch SL3 turned on such that the terminal Pa of the coil A is coupled to the supply voltage source Vm, the terminal Pb of the coil B is floated, and the terminal Pc of the coil C is coupled to the ground potential. As a result, the drive current flows from the supply voltage source Vm, through the coils A to C in sequence, finally to the ground potential. During the third phase, only are the second high-side switch SH2 and the third low-side switch SL3 turned on such that the terminal Pa of the coil A is floated, the terminal Pb of the coil B is coupled to the supply voltage source Vm, and the terminal Pc of the coil C is coupled to the ground potential. As a result, the drive current flows from the supply voltage source Vm, through the coils B to C in sequence, finally to the ground potential. During the fourth phase, only are the second high-side switch SH2 and the first low-side switch SL1 turned on such that the terminal Pa of the coil A is coupled to the ground potential, the terminal Pb of the coil B is coupled to the supply voltage source Vm, and the terminal Pc of the coil C is floated. As a result, the drive current flows from the supply voltage source Vm, through the coils B to A in sequence, finally to the ground potential. During the fifth phase, only are the third high-side switch SH3 and the first low-side switch SL1 turned on such that the terminal Pa of the coil A is coupled to the ground potential, the terminal Pb of the coil B is floated, and the terminal Pc of the coil C is coupled to the supply voltage source Vm. As a result, the drive current flows from the supply voltage source Vm, through the coils C to A in sequence, finally to the ground potential. During the sixth phase, only are the third high-side switch SH3 and the second low-side switch SL2 turned on such that the terminal Pa of the coil A is floated, the terminal Pb of the coil B is coupled to the ground potential, and the terminal Pc of the coil C is coupled to the supply voltage source Vm. As a result, the drive current flows from the supply voltage source Vm, through the coils C to B in sequence, finally to the ground potential.
In the technical field of the sensorless motor control, the timing of commutation is decided on the basis of information provided by the terminal voltage of the floated coil since the potential difference between the terminal voltage of the floated coil and the central coupling point N is a representative of the BEMF of the floated coil. FIG. 2 is a timing chart showing the voltages at the terminals Pa, Pb, and Pc of the coils A, B, and C. As clearly seen from the figure, an abrupt spike occurs immediately after the coil enters floated because of the inductive nature of the coil. For example, in the case of transitioning from the first phase to the second phase, the current originally flowing through the coil B, which is not allowed to be removed immediately, changes its course to flow through the freewheel diode of the second high-side switch SH2 and back to the supply voltage source Vm. Therefore, at the initial period of the second phase, the voltage at the terminal Pb of the coil B makes an abrupt jump higher than the supply voltage source Vm by a forward diode drop. In order to prevent such an abrupt spike at the initial period of coil's floating phase from affecting the determination of the zero-crossing of BEMF, the prior art sensorless motor control circuit intuitively employs a mask circuit to simply disable the comparator circuit at the initial period of coil's floating phase for trying to avoid detecting any of the abrupt spike.
FIG. 3 is a circuit block diagram showing a conventional sensorless motor control circuit 30. A BEMF detector circuit 33 is coupled to the terminals of the coils A, B, and C for sensing the BEMF of the floated coil. A mask circuit 34 is arranged between the BEMF detector circuit 33 and a comparator circuit 35 for preventing the abrupt spike from being delivered to the comparator circuit 35 to cause an erroneous result. Based on the zero-crossing detected by the comparator circuit 35, a drive signal synthesizing circuit 36 generates appropriate drive signals for controlling the high-side and low-side switches of a switching circuit 32 to commutate a motor 31. Moreover, the drive signals may be provided to the mask circuit 34 and the comparator circuit 35, for synchronizing the timing of the mask process with the commutation of the motor 31.
In the prior art, the mask circuit 34 is typically pre-designed with a mask period having a fixed duration, which is presumed long enough for masking all of the abrupt spike at the initial period of coil's floating phase. However, the period of time when the abrupt spike exists actually varies depending on the rotation rate of the motor, the magnitude of the drive current, and other dynamic factors. Even though some of the prior art mask circuits have been improved to provide a mask period having an adaptive length, it is still necessary to choose a particular, fixed rate for adapting the mask period. When the adaptive rate is chosen inappropriately, the abrupt spike may not be completely masked in some cases.